Feel like things are out of sync? Maybe your circadian rhythm isn’t working well.

We all have an intrinsic rhythm that keeps our sleep and other body functions in sync. Understanding how the circadian rhythm works can help get it working well for you and have you feeling your best. There is increasing recognition that when the body clock (circadian rhythm) is not well synchronised, it can impact on a range of health problems, and has significant effects on metabolism, the cardiovascular system and mood.

Video Timeline:

  • 00:00 – 00:56 Internal body rhythms
  • 00:56 – 02:22 The circadian rhythm
  • 02:22 – 05:52 Properties of and problems with the circadian rhythm
  • 05:52 – 09:28 Synchronising the rhythm and peripheral clocks
  • 09:28 – 11:18 Inputs to the circadian rhythm
  • 11:18 – 12:11 Managing the circadian rhythm

Related posts & links:


Hi. I’m David Cunnington. I’m a sleep physician and I’m going to talk about what the circadian rhythm is, so you can get a bit of an understanding about how it works and therefore how it governs lots of body functions.

So, all living things have a number of intrinsic rhythms. Some of those rhythms called ultradian rhythms run for less than 24 hours or within a day, things like mood cycling across the day, sleep cycles within sleep, hunger cycles or feeling full type of cycles.

Then we’ve got cycles that run about a day called circadian cycles and that’s the sleep-wake pattern or the activity and rest type of pattern are common ones with those and then we have infradian rhythms or rhythms that run for longer than 24 hours. Examples of those are say the menstrual cycle that runs over 28 days.

The characteristic of a circadian rhythm which is what I’m talking about here is one that runs across a 24-hour period. It repeats once a day. It persists even in the absence of any external cues. So if someone goes and lives in a cave – in fact that’s how the experiments were done in the 1940s that started to work this out. People went and lived in caves completely removed from light cues or other time cues and showed that they were waking around the same time every 24 hours.

This concept has actually been first described in 1729 by a French researcher who noticed that plants when put in a cupboard would still open when it was light outside. Initially it was thought that those flowers opened because of sunlight but in fact when the plants are in the cupboard, they still opened in the same way, showing that the plants have their own intrinsic rhythm. All living things have these intrinsic rhythms that govern most functions.

The way these rhythms are regulated is there are external factors that synchronise these rhythms and give it a cue that hey, we’re on earth. This is the time to be awake or this is the time to be active or it’s dark outside and this is the time to be quiet and still – and sleeping.

The main cues or inputs to the circadian rhythm are light, physical activity, and food or meals. The circadian rhythm has three key properties. That is the length or period, the amplitude or really think of that as how strong the rhythm is and the phase and that’s how the rhythm relates to the world around us.

In terms of the circadian period for humans, the average length of our intrinsic rhythm is not 24 hours. In fact it’s around 24 hours and 15 minutes yet we live on a world that’s about – not about. Has an exactly 24-hour cycle which means that for most of us, when the alarm goes 24 hours after a time it went the day before, according to our intrinsic clock, it’s going 15 minutes too early.

But because it’s only 15 minutes out, we sort of maybe hit snooze once and feel OK, start getting up. We’re on our feet with some activity, getting some light. That just re-syncs or resets the body clock, pushing it back that 15 minutes. So essentially day after day we feel like we’re running on a 24-hour schedule.

Now for some people though, the body clock is much longer and that can happen in teenagers or adolescents for example where they may have more a 24-hour 30-minute clock or 24-hour 45-minute clock even. It just feels every day like you’re getting up 45 minutes too early and you give into that eventually because it just feels too hard. That can lead to the body clock drifting gradually later and later and cause problems with a delayed phase or sleeping too late.

Another important property of the circadian rhythm is the amplitude or think of that as the strength of the rhythm. If people have a very robust circadian rhythm, you really get that intrinsic sense of there are periods in the day when you feel alert, energetic and couldn’t sleep and other periods in the day when you feel really sleepy and no matter what, you couldn’t even stay awake even if you tried.

Generally people who are in good physical health, good mental health, not taking many medications, maintaining a regular routine and being physically active will have that type of strong circadian rhythm and people who have got problems with physical health or mental health, taking lots of medications, which has done well for a range of reasons often don’t have that sense of that same strong intrinsic drive. They get a sense of, “You know what? At night-time I feel a bit more sleepy than I do during the day and during the day I feel a bit more alert than I do at night,” but there’s not a major difference between those things and things don’t get a sense of rhythm. They don’t get a sense that there’s this day-night, on-off type of cycling across the 24 hours.

The third main property of the circadian rhythm is the phase. I sort of alluded to this in talking about the period or length of the rhythm but that’s how the rhythm relates to the outside world.

Now an example of how that shifts really quickly is we jetlag. All of a sudden we get plunked on the other side of the world. Our intrinsic body clock is staying on our home rhythm but all of a sudden we’re 10 hours out. By the way we have been sort of plunked on the other side of the world and that’s an example of where there’s a problem with the phase.

But a more common problem we see is again that example of teenagers with the long period or long body clock who unless they’re in a regular routine and getting up at a regular time, find they gradually drift out later and later in the holidays, find they are sleeping at 3:00 AM until noon and then it comes back to school and their body clock is on this 3:00 AM until noon pattern but the desired sleep pattern is more a 10:00 PM until 6:00 AM pattern. So they’ve got that six-hour offset and that’s a problem with a phase called delayed phase.

If the circadian rhythm is working well, then essentially everything is working well. We feel like we’re in good health. Our gastrointestinal system is working well. Our blood pressure system is working well. Our temperature system is working well and we’re sleeping well. So we generally feel that we’re well.

An example of things when it gets out of sync is jetlag and you non-specifically just don’t feel quite right when you’ve got jetlagged. It’s essentially all these internal rhythms being desynchronised. So your feeding rhythm might be out of pattern with your sleep-wake rhythm and when your intestines are starting to move things along is out of sync with other rhythms and just doesn’t feel right and takes a few days before things seem like they’re back on track.

So if things are on track and we’re on a regular rhythm, the sleep-wake pattern is also very regular. This is the sleep diary used – or calculated using a technique called “actigraphy” which is similar to a lot of the activity bands that measure movement. Essentially when there’s movement, you see black on the graph and when it’s white, it’s no movement.

So the white or no movement equates to sleep and the blackened movement equates to wake. Each day is plotted as one line and you get seven days stacked on top of each other and it builds up a week. You can see this person has got a really regular rhythm. There’s almost no activity while they’re asleep and lots of activity while they’re awake. A little bit of a sleeping on the weekends, so a bit less activity on the day six and day five in the mornings. That’s a pattern many of us follow. But for many people, if their intrinsic rhythm isn’t working well, they’re not able to have that sense of the body has got this nice rhythm and routine to it.

Rhythm is not just about the central circadian rhythm in the brain. Every single cell in the body in all the different organs in the body has its own clock and we can get into this situation where one clock is out of sync with another clock. I talked about that jetlag example a little earlier and that’s one example.

Some of what we’re beginning to understand and I think we will hear more about in coming years is the importance of synchronisation of peripheral clocks, particularly clocks involved in metabolising energy, absorbing energy from food that we eat and their synchronisation with the central clock because when those things are out of sync, it leads to problems with metabolism.

So if we’re taking meals at times when the peripheral clock is not ready for them, then the liver and other gastrointestinal organs aren’t ready to break down or utilise that energy in quite the same way.

So for example, the body gets a sense of I’ve got all this energy. I wasn’t ready for it. I’m not ready to process it so I’m going to have to store it in a less efficient way, maybe store it as fat, maybe have more glucose circulating in the blood and there is data coming out now showing that this timing of meals and regularity of meals and synchronising food intake with where the peripheral clocks are at is a really important factor in maintaining good metabolic health and a healthy weight.

It’s really relevant because in our modern lifestyles, we’re just increasingly busy and our lives are chaotic and we’re eating on the run and sort of catching bits of food on the go. It may be that part of that lifestyle is actually contributing to some of the problems with obesity in modern society and getting back to a more regular routine where we have predictable meals or predictable times, synchronising those peripheral body clocks, synchronising them with the central clock, getting the body to handle energy efficiently enables us to be healthier.

The key inputs to the circadian system are light. So light not just normal indoor light or what we call a warm light in the red sort of wavelength but particularly broad spectrum white light and in particular within that light in the blue-green wavelength spectrum.

So if we’re using light to give input to the circadian system, outdoor light is by far and away the best. Even on an overcast cloudy day if we’re outside without sunglasses, there’s plenty intensity of light at the right wavelength to synchronise the body clock.

Now the body clock mechanisms are only sensitive to light for an hour or two after we first wake up in the morning or the time when we naturally arise in the morning.

For the rest of the day, light can have an alerting effect but it doesn’t have much of an effect on the body clock. That is until the evening when the sun goes down and it’s dark outside. If we’re getting too much of that bright light at the right wavelength, it will trick the body into hey, it’s not dark yet. It’s not time to start winding down those body systems.

That’s where tablets, computers, things that have quite a high intensity of that blue-green wavelength light held close to the face can make it difficult to shut down and actually push the body clock out later.

Another key input to the circadian system is physical activity. Just movement of the large joints seems to be important in feeding back into the central clock and telling the central clock, hey, I’m moving therefore it must be daytime. It must be time to get going, start firing up all those cognition mood centres. So we feel alert and energetic.

The other key input to the circadian systems I’ve already talked about is meals. So the body takes cues from meals. So the time we’re giving it food, what sort of food we’re giving it, helps also synchronise both the central clock and the peripheral clocks.

When we’re trying to manage the circadian system, so one of the keys to managing it is bearing in mind those different aspects that can feed into the circadian system so in managing light, good exposure to light first thing in the morning, managing light before retiring to bed, managing physical activity, making sure we’ve got movement, particularly soon after getting up to give the body that cue. Hey, time to get going, and with meals, keeping regular and predictable meals at predictable times.

So I hope that has given you an idea of how the circadian system works and there will be a number of other videos talking about circadian rhythm disorders and how to actually manipulate or work on the circadian rhythm to get it working better for you, so look out for those and I will put links at the end of the video post.


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